Current approaches for assessing molecular endpoints in certain diseases usually require tissue and blood sampling, surgery, and in the case of experimental animals, sacrifice at different time points. Despite improvements in non-invasive imaging, more sensitive and specific imaging agents and methods are needed. Imaging techniques capable of visualizing specific molecular targets and/or entire pathways would significantly enhance our ability to diagnose and assess treatment efficacy of therapeutic interventions for many different disease states. Most current imaging techniques report primarily on anatomical or physiological information (e.g., magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound). Newer modalities such as optical imaging and new molecular imaging probes have the potential to revolutionize the way disease is detected, treated, and monitored.
The common paradigm for molecular imaging involves the use of a “molecular” probe or agent that selectively targets a particular gene, protein, receptor or a cellular function, with the absence, presence or level of the specific target being indicative of a particular disease state.
In particular, optical imaging offers several advantages that make it a powerful molecular imaging approach, both in the research and clinical settings. Specifically, optical imaging can be fast, safe, cost effective and highly sensitive. Scan times are on the order of seconds to minutes, there is no need for ionizing radiation, and the imaging systems can be relatively simple to use. In addition, optical probes can be designed as dynamic molecular imaging agents that may alter their reporting profiles in vivo to provide molecular and functional information in real time. In order to achieve maximum penetration and sensitivity in vivo, the choice for most optical imaging in biological systems is within the red and near-infrared (NIR) spectral region (600-900 nm), although other wavelengths in the visible region can also be used. In the NIR wavelength range, absorption by physiologically abundant absorbers such as hemoglobin or water, as well as tissue autofluorescence, is minimized.
Gram negative and gram-positive bacteria are the most common causes of infection such as peri-prosthetic, joint, bone, vascular prosthetic graft, inguinal, umbilical or incisional hernia mesh infections, and one of the challenging complications of surgery. Conventional diagnostic methods relying on the analysis of cultured bacteria recovered from suspected sites are time-consuming, insensitive, and not always feasible. Anatomic imaging techniques such as MRI and CT do not consistently distinguish infection from sterile inflammation and are therefore unreliable. More recently, the use of radiolabeled antibiotics and peptides directed to specific organisms has been studied, but clinical use has been limited, in part because of suboptimal specificity or sensitivity.
Early detection of bacterial infection is correlated with greater prognosis for full recovery. The unique cellular architecture of bacteria presents a number of different avenues for detecting and labeling bacteria at sites of infection. The cellular membrane, DNA and cell wall are three, well-studied bacterial structures. The cell surface of bacteria are highly negatively charged, more so than healthy mammalian cells. Positively-charged cationic probes can be used to selectively target and bind the anionic surfaces of bacterial cells over healthy mammalian cells.
Mammalian cells undergoing apoptosis, or programmed cell death, also express a much higher negative charge than normal cells. Positively charged cationic probes that target bacteria or bacterially compromised cells could also recognize and bind unhealthy, apoptotic mammalian cells as well.
Using such an approach, cationic molecules specific for anionic surfaces, such as those found in bacterial and apoptotic cells, can be linked to fluorescent molecules and used to probe for sites/areas of bacterial infection and/or apoptosis.
The ability to more accurately and efficiently detect and quantify sites of bacterial infection and apoptosis will aid in the understanding of biological phenomena such as pathogenesis, infection and cell death, as well as in the determination of the most appropriate treatment regimens.